This invention relates to hydrogen generation systems for mobile applications and more specifically, to a process of generating hydrogen from a stabilized slurry of alkali borohydride particles in water typically stored at ambient temperatures.
In place of hydrocarbons as a source of energy, hydrogen has been suggested as being a more beneficial alternative. Hydrogen gas can generate more power per gram and emit less, or even no, exhaust pollutants into the atmosphere. However, the difficulty of using hydrogen as a power source is that it is difficult to store, especially in mobile applications.
Hydrogen gas can be stored at high pressure in thick walled vessels. However, the vessels are too heavy, and/or bulky, for vehicles and the high pressure is a concern. Metal hydrides, MHx, contain thermally releasable hydrogen, but only in small, inefficient amounts. Alkali borohydrides contain a higher proportion of hydrogen that can be released by hydrolysis. But this approach, too, is an engineering challenge.
In one system, lithium borohydride particles and water are separately introduced into a pressurized reactor vessel upon demand for hydrogen gas from a fuel cell. The borohydride and water react to form hydrogen and solid by-products. The reactants are thoroughly mixed using paddles or other stirring mechanisms. Heat is applied to initiate hydrolysis and the demanded quantity of hydrogen is then produced.
Another system provides a hydrogen generation system that employs a water-based, alkaline stabilized sodium borohydride solution as a source of hydrogen gas. Upon demand from a fuel cell, the hydride solution, comprising an excess of water, is pumped over a bed of supported ruthenium or other metal catalyst where it hydrolyzes and produces by-products and hydrogen gas. When the fuel cell no longer needs hydrogen, the pumps turns off and the reaction halts. This system requires a significant amount of excess water to prevent by-product covering and deactivation of the catalyst. This limits the hydrogen producing capacity of the system.
Although these borohydride systems provide lower pressure means of storing and generating hydrogen power, improvements are still needed in reducing both the volume and mass of the overall system. Therefore, it is an object of the present invention to provide a hydrogen generation system that is compact, requires a minimal amount of heavy equipment, and does not require an excess of water to produce an optimal yield of hydrogen gas.
This invention provides a method of generating hydrogen to power a hydrogen consuming device. A system for practicing the method is compact and generates a high yield of hydrogen gas. The method uses a slurry comprising alkali borohydride and water with dissolved hydroxide ion stored, for example, on-board a motorized vehicle in a storage tank. The slurry is desirable because there is a relatively high proportion of hydrogen in alkali borohydrides, especially lithium and sodium borohydride, and the contribution of hydrogen from water used in the hydrolysis reaction. By limiting the water content of the slurry to an amount required for the hydrolysis of the alkali borohydride, a higher hydrogen production efficiency can be obtained. The hydroxide operates as a base stabilizer to inhibit the reaction between borohydride ions and water while in storage at normal ambient conditions experienced by the vehicle. For lithium borohydride, a 1 to 5% by weight lithium hydroxide solution is suitable. Likewise, a 1 to 5% by weight of sodium hydroxide solution is suitable for sodium hydroxide slurries.
Upon demand from a hydrogen consuming device, a portion of the slurry is conveyed to a suitable axial flow through reactor where the wetted borohydride particles hydrolyze to produce a demanded quantity of hydrogen gas, water vapor and solid by-products. The slurry must be heated to a temperature of, e.g., 120xc2x0 C. where the hydrolysis reaction is self-sustaining and exothermic. Various heating practices are suitable.
The slurry can be heated prior to entering the reactor vessel using waste heat from the reactor or a supplemental electric heater. In addition, a controllable electric heat source is preferably used to heat the reactor to reach and maintain reaction temperatures of at least 120xc2x0 C. Once initiated, the reactor temperature can usually be maintained by the heat of the hydrolysis reaction. Thus, supplemental heating is used to maintain reaction temperatures only if the amount of heat generated during hydrolysis is insufficient.
As the borohydride particles react with water, hydrogen gas is generated along with solid by-products. The generated hydrogen increases pressure in the flow reactor. The solid by-products include alkali metaborate and hydroxide containing compounds which tend to coat unreacted hydride particles and inhibit further hydrolysis. Any non-consumed water, some in the vapor state, will remain as well.
The stabilized slurry is moved from storage to the reactor via a pump or other suitable two-phase transport system. A suitable seal mechanism is provided at the entrance of the reactor to prevent back flow of hydrogen. When the immediate requirement for hydrogen is satisfied, the pumping of reactants is stopped and the seal mechanism closed.
In a preferred embodiment, the hydrolysis reactor is an elongated cylinder adapted for flow of the lithium or sodium borohydride particles/water slurry from an entrance and to an exit. As hydrolysis occurs in the reactor, the borohydride particles are consumed. Hydrogen gas is, of course, formed along with solid by-product foam masses. The pressure of the hydrogen and gravity (if the reactor is inclined or vertical) help to move the reacting solid mass through the reactor.
To assure reaction of the borohydride particles and to increase the yield of hydrogen gas produced, the reactor vessel preferably includes a tri-functional mixing element. First, the mixing element is a shearing device used to remove by-products coated on the borohydride particles to expose the particles to remaining water and water loosely held in the form of hydrates. Second, the mixing element is a grinding device to crush by-product particle agglomerations into a fine, polydisperse powder. As a powder, by-product waste can be easily and compactly stored in a by-products storage vessel. Third, the mixing element operates as a two-phase mass handling device that transports solid and liquid-phase materials axially through the reactor while allowing hydrogen to exit the reactor. A pair of parallel, closely spaced, counter rotating augers supported between the ends of the reactor provides those three functions.
The delivery of slurry to the reactor is controlled so that a suitable gas space for the formed hydrogen is retained as the augers are moving, mixing and grinding the mixture of solid borohydride and solid foamed by-products. Preferably, the gas space is just as large as the borohydride-water-solid by-product mass.
In one embodiment of the invention, the reaction products are conveyed from the reactor to an unheated by-products storage vessel that includes a gas/vapor space. This vessel facilitates separation of hydrogen from solid by-products. In addition to the mixing element used in the reactor vessel, a supplemental grinding mechanism may be desirable downstream of the reactor to complete crushing of by-product masses before they enter the by-products storage vessel. In this embodiment, the solid-phase components collect in the bottom of the storage vessel while hydrogen gas and any water vapor can be removed from the top. Usually, the hydrogen/water vapor mixture will flow due to the pressure of the generated hydrogen. The temperature of the unheated storage vessel will normally be cooler than the reactor and some of the water vapor generated in the reactor will condense in the by-products storage vessel. In applications where minimizing volume is essential, the by-products may be stored in the same physical vessel as the reactant, separated by a moving divider, or held in two bladders.
Hydrogen is vented from the by-products storage vessel directly to the hydrogen consuming device or to a buffer container. If the pressure of the hydrogen from the by-products storage tank is not high enough to enter the buffer container, a screw compressor or other pressure increasing device may be employed. Thus, the method of this invention uses a storage container for the borohydride slurry, a heated reactor for the hydrolysis of the borohydride and a by-product storage vessel (possibly cooperatively using the storage vessel) for the mixed borate by-products, and for separation of the hydrogen gas demanded by a vehicle engine or other hydrogen consuming device.
As generated, the hydrogen gas usually contains water vapor. Some hydrogen systems may require reduction of the water content. Thus, in another embodiment, the water vapor content of the hydrogen gas from by-product storage is further reduced before the hydrogen is sent to a buffer container. Specifically, the hydrogen and water vapor are directed to the bottom of the slurry storage tank where they bubble through the slurry. The relatively cool storage tank allows some of the water vapor to condense and remain in the slurry. Thus, water needed for the hydrolysis is conserved and drier, lower temperature hydrogen is conveyed to the hydrogen buffer container.
In a third embodiment, a hydrogen generation system is provided that uses a heated flow reactor including both mixing and supplemental grinding functions. This longer reactor vessel includes a grinding mechanism to more completely pulverize by-product masses and expose unreacted borohydride particles to water and hydrates. The reaction products are then conveyed to a by-products storage vessel from the reactor""s outlet. Again, the hydrogen gas and any water vapor flow to either a hydrogen buffer cavity or to the hydrogen consuming device.
In a fourth embodiment, either the reactor vessel outlet or a separate element downstream comprises a gas/solid separation unit to generate two outlet streams; a gaseous-phase stream and a solid-phase stream. The gas stream generated, comprising hydrogen gas and remaining water vapor, flows back into the fuel supply storage vessel where part of the water condenses in the slurry. The hydrogen then continues on to the hydrogen buffer. The solid stream components are conveyed directly to a by-products storage vessel.
In all embodiments, valves controlled at the inlet of the hydrogen buffer may be added to isolate segments when the vehicle or hydrogen user is turned off. This will facilitate either removal or replacement of the hydrogen generating system, or removal of by-products and refueling of slurry with minimal hydrogen release. Furthermore, the practice does have energy requirements in that the reactor must be heated to initiate hydrolysis and mixing and grinding of the solid/liquid/gas mixture is required.
Generally, the present invention involves the transport of thick, wetted solid material into a reactor vessel. The slurry comprises a sufficient amount of water to generate a demanded quantity of hydrogen and is consumed as the hydrogen is generated. The result is a mixture of two phases, where foamy by-products comprising larger particle sizes must be transported out of the reaction zone. Thus, the material exiting the reactor is a thicker mass of by-product material than the slurry that entered the reactor. Therefore, the methods of practicing the present invention require systems that can be adapted to and capable of handling such materials that are relatively difficult to transport.
These and other objects and advantages of this invention will become apparent from a detailed description of the specific embodiments that follow.